The present invention relates to acousto-optic tunable filters and particularly, a design configuration for a non-collinear acousto-optic tunable filter using a crystalline material consisting of either mercurous chloride, mercurous bromide or mercurous iodide.
The term acousto-optic filter refers to the fact that in certain birefringent optical materials, a light beam propagating as an extraordinary ray, can under certain conditions be converted into an ordinary ray (or vice-versa) by interaction with, and diffraction from, an acoustic wave propagating in the same medium. This phenomenon has been utilized in producing narrow band optical filters in which the peak transmission wavelength can be selected by properly choosing the frequency of the acoustic wave.
It is known to use an acousto-optic tunable filter such that optical and acoustic waves propagate collinearly in a suitable birefringent crystal. The light transmittal by the crystal is controlled by the acoustic frequency and power level. Only a very small number of crystal materials can be used in such a collinear device because the required interaction, determined by the photoelastic tensor components, is non-zero for only a few special cases. An example of the aforedescribed collinear configuration is taught in U.S. Pat. No. 3,679,288 to Harris. A non-collinear configuration in an acousto-optic tunable filter is taught by U.S. Pat. No. 4,052,121 to Chang in which the requirements on the crystal properties are relaxed so that a larger number of materials can be utilized.
The development of new efficient infrared acousto-optic materials such as thallium-arsenic-selenide (Tl.sub.3 AsSe.sub.3) as described in U.S. Pat. No. 3,792,287, thallium phosphorus-selenide per U.S. Pat. No. 3,929,970 and thallium-arsenic-sulfide per U.S. Pat. No. 3,799,659 all owned by the assignee of the present invention, provides the possibility of operation over the near-to-mid infrared range of from about 1.3 micrometers to about 16 micrometers.
A large and growing market exists for analytical devices which can be used to analyze reaction products of a wide variety of industrial processes. In addition on-line real time combustion product analyzers are needed to facilitate more efficient burning of hydrocarbon fuels. Multi-function combustion product analyzers are needed to keep track of combustion emission and to ensure that the emission is within the limits set by environmental legislation. On-line multifunction analyzers are needed in the chemical and petroleum industries to function as process analyzers to facilitate process control systems. The petroleum industry as well as the emerging synthetic fuel industry has need for compact, stable and simple analytical instruments for determining hydrocarbon fractions.
The analytical devices to date utilize ultraviolet and infrared spectrophotometry, as well as gas and liquid chromatography in meeting the laboratory and industrial needs outlined above. Such optical instruments as well as the system of the present invention utilize the following important characteristics of materials. A particular molecule has a characteristic absorption spectrum which is dissimilar from that of all other molecules. The spectra of mixtures of molecules are additive and the absorption is proportional to the concentrations of the molecules. Optical absorption spectra can be obtained from any type of sample be it solid, liquid, or gas so long as the sample is optically transmissive, and the spectra can be obtained in a non-destructive testing of the sample.
Infrared radiation is particularly suited for analyzing complex streams of hydrocarbons and combustion products because of the infrared absorption characteristics of the major gases produced in such systems. The existing infrared analytical systems typically are limited to measuring a single wavelength of interest at a given time. Such infrared instruments utilize prisms, selectively absorptive filters, or diffraction gratings so that the filtering mechanism can require mechanical changing of the optical filter element in order to function over a variety of wavelengths. Such mechanical changeovers require realignment of the systems which are time consuming and difficult to achieve at an on-site location.
It is an object of this invention to provide a non-collinear acousto-optic filter for use from the visible through its transmission range in the infrared region of the spectrum to 20 micrometers, with other mercurous halide crystals extending this to 40 micrometers.
It is another object of this invention to provide a non-collinear acousto-optic tunable filter in which the plane of incidence of both the light and the acoustic waves is the plane containing the (001) and (110) crystallographic directions.
It is yet another object of this invention to provide a crystal material for use in the present non-collinear, acousto optic tunable filter system, which crystal material has a tetragonal, Class D (4h) crystal structure.